In the field of manufacturing, frequent measurement and inspection are undertaken to ensure that manufactured parts conform to design specifications. This includes the inspection of holes, such as drilled holes, to ensure that the holes are of the desired shape and configuration, e.g. diameter and alignment, within engineering tolerances. Ensuring conformance of production parts to design specifications is of particular concern in industries such as aerospace manufacturing, where exacting production standards are maintained.
There are many conditions that can produce drilled holes that are out of specification. One common out-of-spec problem for drilled holes is the production of a hole that is oversized. Drill bits can also deflect while drilling, causing holes to be out of profile. Tools that malfunction or are not properly calibrated or adjusted can suffer from chatter, vibration or other problems, which can produce oversized, out-of-round, deflected (e.g. deflected from the drilling axis) or otherwise defective holes. Similarly, dull or damaged bits can produce defective holes. In this regard, robotic or computer numerically controlled (“CNC” or “N/C”) drilling equipment can suffer these and other types of malfunctions that can result in holes that are outside of design tolerances.
To ensure compliance of drilled holes with their design specifications, it has generally been the practice either to have a quality control or quality assurance (“QA”) inspector examine each hole, or to use statistical techniques to analyze a sampled number of holes. Inspection may be performed by manually inserting a hole diameter probe, such as a capacitance-based probe, an air pressure hole probe, a laser hole probe, or a ball-type probe, into a hole to check its compliance with or variation from design specifications. Unfortunately, this process is relatively expensive and time-consuming, and is also dependent upon the skill and training of the inspector.